EP0270920A2 - Method and apparatus to generate emission current signals in an alternating current distribution network - Google Patents

Abstract

A transmitter with controlled switches for use in a process for the generation of transmission current signals in an alternating current distribution network is connected to the supply network. When operated, this transmitter generates a series of current impulses. The moments in time of the generation of these current impulses and/or their duration are so chosen that the resulting transmission signal corresponds at least approximately to a desired theoretical signal. The transmitter contains at least two energy storages or energy storage devices and at least two switches for controlling the energy exchange between the two storages. These switches are so controlled that the exchange current flows through the mains, whereby the aforesaid current pulses are generated. It is thereby possible to use substantially only loss-free elements for the transmitter. The main field of application of the transmitter lies in ripple control technology, where it may be used as a transmitter in a central station (center) or as a transmitter for return signals to the center and for remote reading through the network of distributed meters with return signalling of the state of the meters to a center.

Description

The invention relates to a method for generating transmission current signals in an AC distribution network by means of a transmitter connected to the network and having a controlled switch, wherein a series of current pulses forming the transmission signal is generated by actuating the switch, and the times of their generation and / or their duration can be selected such that the resulting transmission signal corresponds at least approximately to a desired theoretical signal.

A method of this type, which is described in European Patent Application No. 85 108 615.7 (publication number 0 175 863), has the advantage that the frequency-selective coupling circuits required in other known methods are eliminated and are replaced by an essentially aperiodic network. This makes it possible to use the same transmitter for different transmission frequencies and to introduce redundancy into the transmission channel by placing the signals on different frequencies. This considerably increases the security of the transmission.

The said network can basically consist of both reactive and ohmic components. However, a closer analysis shows that, on the one hand, no choke coil may lie directly in series with the switch if the latter is not dimensioned for a very high recurring voltage. On the other hand, no capacitor may be connected directly to the switch, that is, connected to the mains voltage or to another capacitor if the switch is not dimensioned for very high peak currents, which would, however, lead to relatively high additional costs.

If the use of series chokes and parallel capacitors is prohibited due to these circumstances, at least without strong damping ohmic components, then such a network offers only little advantages compared to a simple resistor. However, a resistor would have the disadvantage that it consumes a considerable amount of mains frequency power. It is not so much the cost of the corresponding energy, but rather the heat generated and dissipated that is disadvantageous.

The aim of the invention is to improve and modify the aforementioned method in such a way that essentially only loss-free elements can be used for the transmitter. This object is achieved according to the invention in that the transmitter contains at least two energy stores and at least two switches for controlling the energy exchange between them, and that the energy stores and the switches are connected to one another in such a way that the exchange current flows over the network and thereby generates the current pulses mentioned will.

The invention further relates to a transmitter for carrying out said method with a control logic supplying the control signals for said switches.

The transmitter according to the invention is characterized by a storage choke and by a capacitor connected in series and containing the second energy store.

• Fig. 1 ein Prinzipschaltbild eines ersten Ausführungsbeispiels eines geschalteten Kondensators eines erfindungsgemässen Senders,
• Fig. 2 ein Blockschaltbild eines Senders,
• Fig. 3 ein Prinzipschaltbild eines zweiten Ausführungsbeispiels eines geschalteten Kondensators,
• Fig. 4 ein Diagramm zur Funktionserläuterung,
• Fig. 5 ein Prinzipschaltbild einer ersten Variante von Fig. 3, und
• Fig. 6 ein Prinzipschaltbild einer weiteren Variante von Fig. 3.

The invention is explained in more detail below with the aid of exemplary embodiments and the drawings; show it:

• 1 shows a basic circuit diagram of a first exemplary embodiment of a switched capacitor of a transmitter according to the invention,
• 2 is a block diagram of a transmitter,
• 3 shows a basic circuit diagram of a second exemplary embodiment of a switched capacitor,
• 4 shows a diagram for the explanation of the function,
• Fig. 5 is a schematic diagram of a first variant of Fig. 3, and
• 6 shows a basic circuit diagram of a further variant of FIG. 3.

The transmitter shown in the figures is used to generate current pulses, and a sequence of these current pulses should match a theoretical signal as closely as possible. This can be achieved by a suitable choice of the time of generation of the current pulses and / or of their duration, namely by the time average of the signal formed from the current pulses (determined over an interval which is a multiple the duration of an individual pulse is) can be brought practically into agreement with the mean value of the desired theoretical signal.

In the following figures, various exemplary embodiments of the transmitter used for generating the current pulses mentioned or of its switched capacitor are described.

1 shows a basic circuit diagram of a first exemplary embodiment of such a switched capacitor GC, which is constructed in the manner of a bridge circuit and consists of four branches. The two nodes 1 and 2 are each connected via a line 3 or 4 to the network terminals 5 and 6, between which the network voltage UN lies. Line 3 contains a storage choke SD (choke voltage UL). The two other nodes 7 and 8 of the bridge circuit are connected to one another via a branch 9 having a capacitor C. Each of the four branches of the bridge circuit contains a parallel connection of a controlled switch S1, S2, S3, S4 and a diode D1, D2, D3, D4. The controlled switches S1 to S4 are preferably one-way current valves that can be switched on and off, such as GTO thyristors or power transistors in bipolar or MOSFET technology, and the diodes D1 to D4 are so-called free-wheeling diodes, which ensure that a switch S1 to S4 switches off when it is switched off the energy stored in the storage inductor SD is added positively or negatively to the capacitor voltage UC of the capacitor C, so that there is no excessively high recurring voltage at the corresponding switch. It is conceivable to combine a current valve and the associated freewheeling diode in one component, in the sense of a so-called RLT (= reverse conducting thyristor).

A certain capacitor voltage UC must be present for the generation of the transmission signal. If I (max) is the maximum value of the desired transmission current, f the transmission frequency and UN (max) the peak value of the mains voltage, then the required capacitor voltage is:
UC = UN (max) + 2.pi.fLI (max)

This capacitor voltage UC can be achieved in the following way: With open switches S1 to S4 (or in other words, with blocked valves), the switched capacitor GC represents a full-wave bridge rectifier, which charges the capacitor C to the voltage UN (max). The arrangement shown now enables the capacitor C to be further charged to a higher voltage value by clocking the switches S1 to S4 in such a way that a 50 Hz active power is drawn from the network. However, since switched capacitor GC does not contain any lossy elements, this increasing energy must cause capacitor C to be charged further.

If the mains frequency fN = 50 Hz and î denotes the amplitude of the input current and ûN the peak value of the mains voltage, then the following relationships apply to the input current I, the mains voltage UN, the power P and the energy change W (t) in the switched capacitor GC :
I = î. sin (2.pi.fN.t)
UN = ûN. sin (2.pi.fN.t)
P = UN. I = ûN.î. (sin (2.pi.fN.t)) 2
W (t) = (ûN.î) .t / 2 - (ûN.î) .sin (4.pi.fN.t) / (8.pi.f)

The energy in the capacitor C thus increases monotonously as long as there is a line frequency component in the input current I which is in phase with the line voltage.

2 shows a block diagram of a transmitter according to the invention, which, in addition to the switched capacitor GC and the storage choke SD, has a stage SE for generating the setpoint, a control logic SL, and a setpoint / actual value comparator SI.

At the input 6 is the mains voltage UN, which is supplied to the setpoint generation SE, the control logic SL and the switched capacitor GC. The setpoint value generation SE, which, in addition to the mains voltage UN, the capacitor voltage UC and other control parameters, such as the signal current I *, are supplied, generates the desired transmission signal Is as an analog signal at one input of the setpoint / actual value comparator SI, at the other Input is the image of the transmission current Ie. The desired transmission signal Is can be, for example, a purely sinusoidal signal. The control logic SL now simulates this setpoint by suitably switching the switches (valves) S1 to S4 of the switched capacitor GC by regulating the difference Is-Ie as completely as possible to zero, which is done according to known methods of power electronics, such as time-discrete two-point or Three-point control takes place.

Here you can see why the required initial voltage UC of the capacitor C is required. By providing the necessary voltage, it ensures the maximum required current change in the storage choke SD.

The latter smoothes the input current by integrating the voltage pulses UN-US, where US denotes the voltage generated by the switched capacitor GC.

By appropriately specifying a 50 Hz target current (charging current), the charge of the capacitor C can thus be increased and, if the capacitor voltage is sufficient, a predetermined transmission signal (transmission current) can be simulated. To enable continuous transmission, these two processes can be overlaid in a simple manner by adding the setpoints of charging current and transmission current and outputting them as a common setpoint.

Whether the charge on the capacitor C has to be increased is determined, for example, once per network period by querying the capacitor voltage UC and, depending on the result, charging current is specified or not. Any excessive charging of the capacitor can be reduced by reversing the sign of the charging current.

The following Table 1 gives an overview of the permissible switching states of the switches and diodes S1 to S4 or D1 to D4 of the switched capacitor GC (FIG. 1) of the transmitter and the behavior of the input current I, the capacitor current IC, the capacitor voltage UC and the Choke voltage UL.

The following symbols are also used in Table 1:
0: valve / diode blocks
1: Valve / diode conducts
x: valve does not conduct due to physical conditions, can be switched on
s: signal value increases
f: signal value falls
=: Signal value constant
g: greater than zero
k: less than zero

There are essentially three different equations for current and voltage:
UL = UN + UC with IC = -I
UL = UN - UC with IC = + I
UL = UN with IC = zero
From these equations and the relationship dI = 1 / L. UL.dt the current forms of the two or three-point control can be determined.

The size of components L and C depends on the permissible values of voltage loading of the valves, current ripple, etc. Useful values are, for example: î = 10 A, L = 1 mH, C = 1 mF.

In Fig. 3, a switched capacitor GC * is shown, which consists of an anti-parallel connection of two branches, each with a controlled switch (valve) S1, S2 and a capacitor C1 or C2 in series. An appropriate flow of switches S1 and S2 results in an energy flow between the network and the two capacitors C1, C2, and vice versa.

Analogous to Table 1, Table 2 gives an overview of the choke voltage UL resulting from the various switch positions of switches S1, S2 depending on input current I and line voltage UN, as well as the behavior of capacitor voltages UC1 and UC2. The symbols used correspond to those in Table 1.

If the capacitor voltages UC1 and UC2 are greater than UN (max) and UN is greater than zero, and for example S2 is ignited for a period of time T1, then the LC resonant circuit is triggered and a current flows from a value I1 approximately linearly increases a value I2. The following applies to the current change: dI / dt = (UN + UC2) / L.

If after the time T1 the valve S2 is extinguished for a time T2, the storage choke SD ensures a further circulation of the current, C2 discharges and C1 charges because D1 is now conducting. The current drops linearly from the value I2 to the value I1, but with a different slope. Trapezoidal current impulses with different energy contents are created. Now the current change is dI / dt = (UN - UC1) / L.

The energy flow taking place via the network between the storage choke SD and the capacitors C1 and C2 and vice versa can be controlled by the length of the current pulses, that is to say by the length of the time periods T1 and T2. If cases b and c are preferred, i.e. longer pulse times are selected for these cases than during a and d, the voltage at C1 and C2 increases. If too high a voltage arises at the capacitors C1 and C2, this can be reduced by giving preference to cases a and d over b and c.

As mentioned in the introduction to the description, the possibility of using the same transmitter for different transmission frequencies without tuning is one of the main advantages of this type of transmitter, because it enables redundancy to be used in the transmission channel. The broadcast of Information referred to as telegrams can basically be of the same content, but with different frequencies, either successively or simultaneously.

In the first case, the setpoint generation SE (FIG. 2) is designed such that it generates a purely sinusoidal signal which is modulated, for example, in amplitude, frequency or phase. In the second case, that is to say when several telegrams are transmitted simultaneously on different frequencies, the setpoint value generation SE is carried out in such a way that it generates the sum of the signals of the different telegrams, the transmitter approximating this setpoint value by means of a series of current pulses. Since the peak value of the transmission current is higher in this case than in the first, the elements of the transmitter must be dimensioned accordingly.

The storage choke SD shown in the figures is preferably designed as a coil with an iron core and an air gap when the transmitter is installed in a low-voltage network. When installing the transmitter in a high-voltage network, or when designing switches S1 to S4 for relatively low voltages, it is advantageous to replace the storage choke SD with the leakage inductance of a transformer. In the case of the formation of the switched capacitor GC according to FIG. 1, the primary winding of this transformer would come to lie between the terminals 5 and 6 and the secondary winding between the terminals 1 and 2.

4 shows, using a diagram, how the transmission signal composed of the individual current pulses can be brought into agreement with the desired theoretical signal.

In the example shown, the desired transmission signal Is is sinusoidal and the transmission current Ie is composed of individual current pulses In. A single current pulse corresponds to the hatched area In. The desired transmission signal Is is sampled periodically and it is determined for each sample whether the current value of the respective current pulse In is greater or less than the corresponding discrete value of Is. Based on the result of this comparison and taking into account whether the respective current pulse In If the next current pulse rises or falls, the switches S1 to S4 or S1, S2 are selected accordingly in accordance with Tables 1 and 2.

In order to achieve a better approximation of the desired theoretical signal Is by the transmission signal Ie consisting of the current pulses In, it can be advantageous in certain cases to reduce the current increase dI / dt and at the same time also the voltage US on the switched capacitor GC. This can be achieved by temporarily having only the mains voltage UN in this circuit.

In these cases, a short circuit is sufficient to achieve the required steepness of the current change in the storage inductor SD, which can be achieved in the switched capacitor GC of FIG. 1 by simultaneous ignition of the switches S1 and S3 or S2 and S4, in each case a short circuit via the storage choke SD. If this short circuit is interrupted, the energy stored in the storage inductor SD goes via the freewheeling diodes D1 to D4 into the capacitor C or into the capacitors C1, C2.

3, such a short circuit cannot be easily realized, but, as shown in FIG. 5, requires a third branch 10 arranged between the two antiparallel branches with a two-way switch (two-way valve) S5 that can be switched on and off.

FIG. 6 shows a variant of the switched capacitor GC * shown in FIG. 3, in which, as shown, a capacitor CS is arranged between the storage inductor SD and the switches S1, S2. This modification, which can of course also be done with the switched capacitor GC of FIG. 1, can be used to achieve a reduction in the switch voltage by pulling the switches S1, S2 by means of a suitable keying, a 50 Hz reactive current can be drawn from the capacitor CS such a drop in voltage causes only a reduced voltage to develop at the switches. The voltage at the capacitors C1 and C2 can be controlled by setting a corresponding 50 Hz active current component.

The voltage at the switches S1, S2 is composed of the capacitor voltages UC1, UC2, the mains voltage UN, and the 50 Hz voltage drop that occurs at the series connection of the storage choke SD and the capacitor CS. If the ignition and extinction torques of switches S1, S2 are set so that a 50 Hz reactive current circulates, the capacitor voltages UC1 and UC2 will not change on average, as can be determined from Table 2. This 50 Hz reactive current generates in the series LC circuit of storage choke SD and capacitor CS a 50 Hz voltage drop, so that only a small remainder of the 50 Hz voltage of the network is produced at switches S1, S2.

Due to the additional capacitor CS arranged in series with the storage choke SD, the switched capacitor GC or GC * from FIG. 1 or FIG. 3 can be operated with switches S1 to S4 or S1, S2 which do not have the full voltage UN + UC or UN + UC1, UC2 need to be dimensioned. This can be particularly advantageous for those transmitters that are connected to the medium voltage. However, precautions must then be taken so that non-transient processes in some way destroy the switches.

As is proven in the introduction to the description of EP-publication 0 175 863 cited at the outset, transmitters of the type described are used in particular for the feedback of the counter status of, for example, electricity meters or the execution of commands from ripple control receivers in a control center. Transmitters of the type described in Swiss Patent Application No. 03 923 / 86-0 are used as ripple control transmitters for the transmission of ripple control telegrams from the head office to the ripple control receivers distributed over the network.

The transmitter described can now be used, as described, for the information transmission in the direction of the decentralized ripple control receiver center and, on the other hand, opens up the interesting possibility of using it as a ripple control transmitter for information transmission in the direction of the center - decentralized receiver. This would result in Known ripple control transmitters have the advantage that the control frequency is essentially independent and that no direct current circuit would be required. However, compared to the static frequency inverters with a DC link, much larger valves would be required because the product peak current times recurring voltage would be larger.

In such an application as a ripple control transmitter, the storage choke GD could advantageously be formed by the short-circuit impedance (= leakage impedance) of a conventional medium-voltage / low-voltage transformer (for example 20 / 0.4 kV). The transmitter would be three-phase, that is to say three-phase single-phase.

Claims (17)

1. A method for generating transmission current signals in an AC distribution network by means of a transmitter connected to the network and having a controlled switch, wherein a series of current pulses forming the transmission signal is generated by actuating the switch and the times of their generation and / or their Duration selected so that the resulting transmission signal corresponds at least approximately to a desired theoretical signal, characterized in that the transmitter at least two energy stores (SD; C, C1, C2) and at least two switches (S1 to S4) for controlling the energy exchange between them contains, and that the energy storage device and the switches are connected to one another in such a way that the exchange current flows through the network and the current pulses (In) mentioned are thereby generated. 2. The method according to claim 1, characterized in that the resulting transmission signal (Ie) is formed by a series of a plurality of current pulses (In) arranged directly next to one another. 3. The method according to claim 2, characterized in that the current pulses (In) have an approximately trapezoidal or triangular shape, so that the resulting transmission signal (Ie) has a sawtooth-like contour. 4. The method according to any one of claims 1 to 3, characterized in that the one energy store is formed by a storage choke (SD) and the other energy store by at least one capacitor (C, C1, C2) arranged in series therewith, and that Energy exchange between the storage choke and the at least one capacitor via the network is controlled by the on / off times of the switches (S1 to S4) and thus the amplitude and duration of the resulting current pulses (In). 5. The method according to claim 4, characterized in that the voltage (UC, UC1, UC2) on the respective capacitor (C, C1, C2) is kept in the vicinity of a target value in that the switches (S1 to S4) are controlled that a positive or negative 50 Hz active current is added to the desired signal current. 6. The method according to claim 4, characterized in that a series capacitor (CS) is arranged in series with the storage inductor (SD). 7. The method according to claim 6, characterized in that the switches (S1 to S4) are controlled such that a 50 Hz reactive current is added to the desired signal current and a 50 Hz voltage drop occurs along the series capacitor (CS), so that the Switches can be designed for a smaller recurring voltage than the sum of the mains voltage (UN) and capacitor voltage (UC, UC1, UC2). 8. The method according to claim 7, characterized in that if the course of a current pulse (In) is too steep, a short circuit is brought about via the storage inductor (SD) and thus only the mains voltage (UN) is used for generating the current pulses. 9. The method according to claim 1, characterized in that the simultaneous transmission of transmission signals of different frequencies, the sum of the different transmission signals is generated and approximated by the transmitter by means of a series of current pulses. 10. Transmitter for performing the method according to claim 1, with a control logic supplying the control signals for said switches, characterized by a storage choke (SD) and by a series-arranged capacitor (GC, GC *), which is the second energy store (C, C1, C2) contains. 11. Transmitter according to claim 10, characterized in that the storage choke (SD) is formed by the leakage impedance of a transformer. 12. Transmitter according to claim 10 or 11, characterized in that the switched capacitor (GC) consists of a bridge circuit, in the branches of which a controlled switch (S1 to S4) and in the diagonal (9) of which a storage capacitor (C) is arranged . 13. Transmitter according to claim 10 or 11, characterized in that the switched capacitor (GC *) by an anti-parallel connection of two Branches with a controlled switch (S1, S2) and a capacitor (C1, C2) are formed in series. 14. Transmitter according to claim 12 or 13, characterized in that the controlled switches (S1 to S4) are formed by on / off switchable one-way flow valves. 15. Transmitter according to claim 12 or 13, characterized by a series capacitor (CS) arranged in series with the storage inductor (SD). 16. Transmitter according to claim 13, characterized in that a third branch (10) with an on / off switchable two-way valve (S5) is provided between the two anti-parallel branches. 17. Transmitter according to claim 11, characterized in that the transmitter is formed in three phases when used as a ripple control transmitter in a center.

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